CEST2 29-31 October 2019 - Sabratha –Libya

2nd Conference for Engineering Sciences and Technology -

CEST2 29-31 October 2019 - Sabratha –Libya

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ANALYSIS OF NATURAL GAS PROPERTIES

(A CASE STUDY OF WAFA GAS FIELD)

Abrahim Qarash 1*, Ahmed Atia 2 , Azden ElShoboky 3

1 [email protected], 2 [email protected], 3 [email protected]

1,2 Department of Petroleum Engineering, Faculty of Natural Resources, Zawia University, Libya 3 Department of Industrial Engineering, Faculty of Engineering, Sabratha University, Libya

A b s t r a c t

The field is located about 540 KM southwest of Tripoli .The southern part was discovered in 1964

by Shell – Libya with well D1, and the northern culmination, was discovered Sirte oil Company in

1991 with well A1. In designing gas production, processing, transport and handling systems, a

complete knowledge of natural gas properties is crucial. For this reason, the purpose of this study

was measurement and prediction of hydrocarbon fluid properties .This study was conducted to

shows the reservoir of natural gas properties analysis of Wafa gas field from gas composition.

Based on the studied composition, it is shown that the Wafa gas is a sweet gas, the viscosity of

the gas is 0.01095 cp at reservoir formation temperature 80.28 ℉and pressure 514.88 psia , The

gas compressibility factor, real gas density and gas formation volume factor are 0.92, 1.80

lbm ft3⁄ , 0.02699 ft3 scf⁄ , respectively.

Key words: Composition, Specific Gravity, properties, Compressibility and Viscosity

1. INTRODUCTION

The field is located about 540 KM southwest of Tripoli and about 160 KM south of the city of Ghadames

in the NC169A concession, along the Libyan – Algerian border. The southern part was discovered in

1964 by Shell – Libya with well D1, and the northern culmination, was discovered Sirte Oil Company in

1991 with well A1. The Wafa Field is a monocline dipping gently towards the Northwest and is a

stratigraphic pinch-out trap formed by the F3 sandstone member of the Aouinet – Ouenine Formation of

Middle Devonian disappearing into equivalent shalyfacies. The field is a gas-condensate reservoir with a

thin oil leg. The average daily production from Wafa field is 37,290 Bbls of crude oil and

condensate,22.503 Bbls of NGL [1]. the natural gas is a mixture of hydrocarbon and non-hydrocarbon

gases. The non-hydrocarbon gases include carbon dioxide, hydrogen sulfide, and nitrogen. natural gas

properties vary significantly with pressure, temperature, and gas composition. Gas properties include

gas-specific gravity, gas psuedo-critical pressure and temperature, gas viscosity, gas compressibility

factor, gas density, gas formation volume factor, and gas compressibility. The first two are composition

dependent. The latter four are pressure dependent.[2 ].All these properties are necessary in the oil and gas

industry for evaluating newly discovered gas reservoirs, calculating initial gas reserves, predicting future

gas production and designing production tubing and pipelines [3].

properties of natural gas play very important roles in gas production. thus, knowledge of petroleum

properties is crucial.[4] These fluid properties are usually determined by laboratory experiments

2nd Conference for Engineering Sciences and Technology -CEST2 29-

31 October 2019 - Sabratha –Libya

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of the gas [5], it is necessary for the petroleum engineer to determine the properties from generalized

mathematical expressions. The objective of the present study is to natural gas properties analysis of Wafa

gas field.

2. MATERIALS

The Composition of Natural Gas

Hydrocarbon molecular weight ,mole fraction, pseudo-critical temperature and pressure for each

component [6] and gas composition of Wafa field have been used for fluid properties estimation which

is listed in Table 1. Reservoir formation temperature (T) and pressure (P) are 26.28 ℃ and 3.4 MPa

have been used for reservoir Wafa fluid analysis. In the present research to complete the analysis

properly, some recognized charts or correlations developed based on the charts have been used.

TABLE 1 critical properties and Mole fraction of defined component.

Component Molecular

weight

Critical

pressure 𝑃𝑐𝑖

(Psia)

Critical

Temperature 𝑇𝑐𝑖

(R0)

Mole

fraction

YI

𝑐1 16.04 343 668 0.86071

𝑐2 30.07 550 709 0.09953

𝑐3 44.10 666 617 0.01442

i𝑐4 58.12 735 530 0.00032

n𝑐4 58.12 765 551 0.00028

i𝑐5 72.15 830 490 0.000

n𝑐5 72.15 845 487 0.000

𝑐6 86.18 913 434 0.000

𝐻𝑒 4.00 9 33 0.00094

𝑂2 31.99 279 737 0.000

𝐻2 2.02 60 188 0.000

𝑁2 28.01 227 493 0.00483

𝐶𝑂2 44.01 548 1073 0.01898

𝐻2𝑆 34.08 673 1306 0.000

A. The Apparent molecular weight (𝑴𝒂) and Gas Specific Gravity:

One of the main gas properties that is frequently of interest to engineers is the apparent molecular

weight. If the mole fraction and molecular weight of each component are given. The Apparent molecular

weight (Ma) has been estimated from the product of each gas component (yi) and molecular weight (Mi)

i.e. Ma = yi ∗ Mi [6] as shown in table 3.

“Specific gravity gas” is defined as the ratio of the apparent molecular weight of the gas to that of air.

The molecular weight of air is usually taken as equal to 28.97 (79% nitrogen and 21% oxygen).

Therefore [2].thus, The specific gravity (γg) is calculated from apparent molecular weight i.e.( γg =

Ma /28.97) .



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For gas mixtures, the gas critical pressure and temperature are called pseudo critical pressure and

temperature to be distinguished from those of pure components, and can be calculated as [7]:

𝑃𝑝𝑐 = ∑ 𝑦𝑖𝑃𝐶𝑖𝑛𝑖=1

𝑇𝑝𝑐 = ∑ 𝑦𝑖𝑇𝐶𝑖𝑛𝑖=1 (1)

where PCi and TCi are critical pressures and temperatures of individual components, respectively. The

reduce pressure and temperatures are:

𝑃𝑝𝑟 =𝑃

𝑃𝑝𝑐

𝑇𝑝𝑟 =𝑇

𝑇𝑝𝑐 (2)

C. Gas compressibility factor and Gas Density:

Gas compressibility factor is also called “deviation factor” or “z-factor.” Its value reflects how much the

real gas deviates from the ideal gas at a given pressure and temperature. Very often the z-factor is

estimated with the chart developed by Standing and Katz (1954). This chart has been set up for computer

solution by a number of individuals. Brill and Beggs (1974) yield z-factor values accurate enough for

many engineering calculations. Brill and Beggs’ z-factor correlation is expressed as follows [2]:

𝐴 = 1.39(𝑇𝑝𝑟 − 0.92)0.5

− 0.36𝑇𝑝𝑐 − 0.10 (3)

𝐵 = (0.62 − 0.23𝑇𝑝𝑟)𝑃𝑝𝑟 + (0.066

𝑇𝑝𝑟−0.86− 0.037) 𝑃𝑝𝑟

2 + 0.32𝑃𝑝𝑟

6

10𝐸 (4)

𝐶 = 0.132 − 0.32 𝑙𝑜𝑔( 𝑇𝑝𝑟) (5)

𝐷 = 10𝐹 (6)

𝐸 = 9(𝑇𝑝𝑟 − 1) (7)

𝐹 = 0.3106 − 0.46𝑇𝑝𝑟 + 0.1824𝑇𝑝𝑟2

(8)

𝑍 = 𝐴 +1−𝐴

𝑒𝐵 + 𝐶𝑃𝑝𝑟𝐷 (9)

The gas density (ρg) is defined as mass (m) per unit volume (V). It has been calculated as following

equation

𝜌𝑔 = 𝑀𝑎𝑃

𝑧𝑅𝑇= 2.7

𝑝 𝛾𝑔

𝑍 𝑇 (10)

where m is mass of gas and ρg is gas density. Taking air molecular weight 29 and R is 10.73 psi-ft3/lb-

mol-R



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Gas formation volume factor is defined as the ratio of gas volume at reservoir condition to the gas

volume at standard condition, that is [2]:

𝐵𝑔 =𝑉

𝑉𝑠𝑐=

𝑃𝑠𝑐

𝑃

𝑇

𝑇𝑠𝑐

𝑍

𝑍𝑠𝑐= 0.02829

𝑍 𝑇

𝑃 (11)

Viscosity is a measure of a fluid’s internal resistance to flow. The viscosity of a natural gas, expected to

increase with both pressure and temperature, is usually several orders of magnitude smaller than that of

oil or water; and therefore, gas is much more mobile in the reservoir than either oil or water. Gas

viscosity correlations have been presented by a number of authors. such as , the Carr, Kobayashi, and

Burrows (1954) correlations. The most commonly used unit of viscosity is the centipoises (cp). Gas

viscosity correlations have been presented by a number of authors. However, the Carr, Kobayashi, and

Burrows (1954) correlation, has been the most popular. calculation the viscosity at any temperature and

at a pressure of 1 atm from Figure 1 . estimation of μg μ 1⁄ , which is the ratio of the viscosity at an

elevated pressure to the viscosity at 1 atm from Figure 2.

3. METHODOLOGY OF GAS PROPERTIES ANALYSIS:

A. Process of Estimating gas compressibility factor and Gas Density:

Steps of Estimating gas compressibility

Determine the pseudo-critical pressure and temperature from Equation 1

Calculate the pseudo-reduced pressure and temperature by applying Equation 2

Determine the z-factor from Equation 9.

Steps of Estimating Gas Density:

calculate the apparent molecular weight Ma

Calculate the specific gravity γg

Determine the Gas Density ρg from equation 10

B. Process of Estimating the Gas Formation Volume and Viscosity:

The gas formation volume factor (Bg) is defined as the volume of gas at reservoir conditions required to

produce one standard cubic foot of gas at the surface. The gas formation volume factor has been

estimated as the following steps [3,6]:

Steps of Estimating Gas Formation Volume:

Calculation the pseudo-critical temperature 𝑇𝑝𝑐 and pseudo-critical pressure 𝑃𝑝𝑐,

Determination of the pseudo-reduced temperature 𝑇𝑝𝑟 and pseudo-reduced pressure 𝑃𝑝𝑟,

Estimation of the gas compressibility factor (Z) from equation 6.

Calculation of gas formation volume factor from equation 11.

steps of Estimating Gas viscosity:

The viscosity of the gas is often estimated with graphs or links developed based on diagrams, in this

study ). The viscosity of mixture gas was estimated from [2]:




𝜇𝑔 =𝜇1

𝑇𝑃𝑟𝑒𝜇𝑟 (12)

The atmospheric pressure viscosity (μ1) can be expressed as:

𝜇1 = 𝜇1𝐻𝐶 + 𝜇1𝑐𝑜2+ 𝜇1𝐻2𝐶 + 𝜇1𝑁2

(13)

Where:

𝜇1HC = 8.188 × 10−3 − 6.15 × 10−3 log(𝛾𝑔) + (1.709 × 10−5 − 2.062 × 10−6𝛾𝑔)𝑇 (14)

𝜇1𝐶𝑂2= [6.24 × 10−3 − 9.08 × 10−3 log(𝛾𝑔)] 𝑦𝐶𝑂2

(15)

𝜇1𝐻2𝐶 = [3.73 × 10−3 + 8.49 × 10−3 log(𝛾𝑔)] 𝑦𝐻2𝑆 (16)

𝜇1𝑁2= [9.59 × 10−3 + 8.49 × 10−3 log(𝛾𝑔)] 𝑦𝑁2

(17)

𝜇𝑟 = 𝑎0 + 𝑎1𝑃𝑃𝑟 + 𝑎1𝑃𝑃𝑟2 + 𝑎3𝑃𝑃𝑟

3 + 𝑇𝑃𝑟(𝑎4 + 𝑎5𝑃𝑃𝑟 + 𝑎6𝑃𝑃𝑟2 + 𝑎7𝑃𝑃𝑟

3 )

+ T𝑃𝑟2 (𝑎8 + 𝑎9𝑃𝑃𝑟 + 𝑎10𝑃𝑃𝑟

2 + 𝑎11𝑃𝑃𝑟3 ) + T𝑃𝑟

3 (𝑎12 + 𝑎13𝑃𝑃𝑟 + 𝑎14𝑃𝑃𝑟2 + 𝑎15𝑃𝑃𝑟

3 )

where:

Process of Estimating the Gas Viscosity with graphs:

Determine the viscosity of the gas from chart [8] at 1 atmospheric(μ1 ) (atm) pressure in Figure-2.

To take into account the effect of the presence of non-hydrocarbon gases using Figure-2.

Estimation of the Viscosity ratio (μ𝑔 μ 1⁄ )at elevated temperature-pressure from chart[8,2] in Figure-3.

Determine viscosity of mixture gas from (𝜇 = μ𝑔 μ 1⁄ ∗ μ 1).

a0 = −2.46211820

a1 = 2.970547414

a2 = −0.28626405

a3 = 0.00805420

a4 = 2.80860949

a5 = −3.49803305

a6 = 0.36037302

a7 = −0.01044324

a8 = −0.79338568

a9 = 1.39643306

a10 = −0.14914493

a11 = 0.00441016

a12 = 0.08393872

a13 = −0.18640885

a14 = 0.02033679

a15 = −0.00060958




Figure1 Viscosity chart at 1 atmospheric pressure (Carr et al, 1954 and Economides et al, 1994)[2].

Figure 2 Viscosity chart at elevated temperature-pressure (Carr et al, 1954 and Economides et al, 1994)[2].

4. RESULTS AND DISCUSSIONS

The gas composition is known and Kay’s mixing rule has been used to calculate the pseudo-critical

temperature & pressure. From the data analysis, a detailed reservoir gas properties results are shown in

Table-2, 3 and 4.In Wafa field, the gas composition of methane (mole fraction) is 0.86071 and H2S mole



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indicate a sweet gas reservoir [9].

TABLE 2 Estimated physical properties of natural gases

The gas specific gravity is 0.64 and API gravity is 89.6. The gas Z-factor has been estimated from . Brill

and Beggs(1974)" z-factor" correlation[2] of 0.94 for sweet gas reservoir. Gas compressibility factor may

be changed if it is estimated by others methods and correlations. The estimated pseudo-critical

temperature and pressure are 372 R0 and 678 psia, respectively. The apparent molecular weight is

18.44.The formation volume factor (Bg) is 0.02699 (ft3 scf⁄ ) at reservoir formation temperature 539.946

R0 and pressure 514.88 psia, respectively. The gas compressibility factor, gas density, gas formation

volume factor are pressure dependent. The viscosity of hydrocarbon gases is 0.01065 cp at 1 atmosphere.

The viscosity of all gases is 0.01076 cp including the effect of the presence of non-hydrocarbon gases at

1 atmosphere. The viscosity of mixture gas is 0.0.01095 cp at mentioned reservoir temperature and

pressure. The effect of each of the non-hydrocarbon gases is to increase the viscosity of the gas mixture.

Gas viscosity decreases with reservoir pressure decreases and vice versa. Viscosity of natural gases

depends among reservoir temperature, pressure and gas compositions [6].

5. CONCLUSIONS

Wafa gas field is a sweet gas reservoir. The gas compressibility factor, real gas density and gas

formation volume factor are 0.92, 1.80 lbm ft3⁄ , 0.02699 ft3 scf⁄ , respectively. , the viscosity of the gas

is 0.01095 cp at reservoir formation temperature 80.28 ℉ and pressure 514.88 psia. The estimated fluid

properties are reliable and can be used for reserve estimation and well test analysis of Wafa gas field.

Properties

Wafa fluid

Reservoir Temperature (R0) 539.946

Reservoir pressure (psia) 514.88

Molecular weight 18.44

Specific gravity 0.64

Tpc(R0) 372

Ppc(psia) 678

Tr 1.45

Pr 0.76

Z-factor 0.92

Real gas density, ρg

(lbm ft3⁄ ) 1.809

Gas formation volume factor, Bg

(ft3 scf⁄ ) 0.02699

Viscosity of gases, μg

(cp) 0.01095




TABLE 3 Kay’s Mixing and pseudo-critical properties of natural gases

REFERENCES

[1] https://mellitahog.ly/en/sites/wafa-field/ [Accessed on 2/8/2018].

[2] Boyun Guo, Xinghui Liu and Xuehao Tan, "Petroleum Production Engineering " 2th,Gulf Professional Publishing,(2017) pp 23-28.

[3] Kumar, S., “Gas Production Engineering”, Gulf publishing Co. V. 4, Houston, Texas, p. 646, (1987).

[4] Economides, J. M. et al., “Petroleum Production System”, Prentice-Hall PTR, p. 611, (1994).

[5] Tarek A (2006) Reservoir engineering Handbook. (3rd edn), Elsevier,USA.

[6] McCain, W. D. Jr., “The Properties of Petroleum Fluids”, second edition, Penn Well Publishing Co., Tulsa, Oklahoma, p. 548, (1990).

[7] Xiuli Wang and Michael Economides, Advanced Natural Gas Engineering, Gulf Publishing Company, Houston, Texas,p.12, (2009).

[8] Carr, N. L., Kobayashi, R. and Burrows, D. B., “Viscosity of Hydrocarbon Gases under Pressure”, Trans., AIME, 201, p. 264-272, (1954).

[9] Imam, B., “Energy Resources of Bangladesh”, First Edition, University Grants Commission Publication No. 89, Bangladesh, p. 280, (2005).

yiPci yiTci Miyi1 Mole fraction

Molecular

weight Gas composition

Ma yi Mi

574.95 295.22 13.81 0.86071 16.04 c1

70.57 54.72 2.99 0.09953 30.07 c2

8.90 9.60 0.64 0.01442 44.10 c3

0.17 0.24 0.02 0.00032 58.12 ic4

0.15 0.21 0.02 0.00028 58.12 nc4

0.00 0.00 0.00 0.000 72.15 ic5

0.00 0.00 0.00 0.000 72.15 nc5

0.00 0.00 0.00 0.000 86.18 c6

0.03 0.01 0.00 0.00094 4.00 He

0.00 0.00 0.00 0.000 31.99 O2

0.00 0.00 0.00 0.000 2.02 H2

2.38 1.10 0.14 0.00483 28.01 N2

20.37 10.40 0.84 0.01898 44.01 CO2

0.00 0.00 0.00 0.000 34.08 H2S

Ppc=678 Tpc=372 18.44 1.0 Total

https://mellitahog.ly/en/sites/wafa-field/

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